1. Field of the Invention
The present invention relates to the use of high purity metal sulfonic acids for use in energy storage devices, methods for preparing high purity metal sulfonic acids electrolytes, methods for efficiently using the metal sulfonic acid solution and products formed by using such methods and solutions. More particularly, the invention provides sulfonic acid solutions that have high cathode efficiencies for 2B series metals such as zinc deposition processes, sulfonic acid solutions that affords high solubility to lanthanide series ions and sulfonic acid solutions that have low concentrations of low-valent sulfur compounds or higher valent sulfur compounds susceptible of reduction that are capable of producing an unwanted odor during electrolysis.
2. Prior Art
Electrochemical processes are used in many large-scale stationary energy device storage applications. The rating of an energy storage device is dependent upon the overall power supply and the discharge time. Power supply can vary from 1 kilowatt (1 kW) in metal-air type batteries to greater than 1 gigawatt (1 GW) in pumped hydro-type batteries. Discharge times may also vary from a fraction of a second to greater than several hours.
Improved reliability or power quality of energy storage devices may require virtually uninterrupted power supplies (UPS). Such electrical storage processes include capacitors, and super conducting magnetic energy storage devices. During a power interruption, these devices are used within fractions of a second to ensure an uninterrupted source of power.
Storage devices may also be used from a few seconds to several minutes in cases where the power is switched from one main power supply (e.g., a power grid) to another main power supply.
Electrical energy storage devices may also be used to provide cost savings to large users in times of limited available power. Such power devices provide sufficient electrical energy from minutes to hours in duration to meet peak energy demands, or to provide a reservoir of power for use in off-peak times.
The capability of the energy storage process and the accompanying power output is dependent upon the engineering design of the device, composition of the electrolytes used in such device and the type of power rating needed. There are several commercial energy storage processes available each with its own advantages and disadvantages. These include the polysulfide bromide battery (PSB), the vanadium redox battery (VRB), the zinc bromine battery (ZnBr), the sodium sulfide battery (NaS), lithium ion battery, compressed air energy storage (CAES), large-scale lead acid battery (LSLA), pumped hydro, E.C. capacitors and flywheel technologies.
There are three flow-type batteries, PSB, VRB and ZnBr. The PSB battery employs two sodium electrolytes, sodium bromide and sodium sulfide. The redox potential of this cell is about 1.5 V and the efficiency is approximately 75%. The VRB use two cell each containing vanadium. In one cell, V+2/V+3 is used and in the other, V+4/V+5. The redox voltage is about 1.4 to 1.6 V and the efficiency is slightly higher than the PSB battery, about 85%. The ZnBr redox potential is about 1.8 V yet the efficiency is only about 75%. All these flow-type batteries have relatively high power ratings and may be used in energy management type applications where supplemental power is needed over an extended period of time. All these flow-type batteries suffer from low energy density.
The NaS battery uses molten sulfur and molten sodium to produce a redox voltage of about 2 volts at an efficiency of approximately 88%. The main drawbacks to this type of battery are the high temperature, 300° C., necessary to keep the metals molten, safety concerns using these materials and high production costs.
The lithium-ion battery has efficiency near 100% and high energy density and long life cycles. Although useful for small-scale applications, the main drawback to large-scale use is the inherently high cost, about $600/kWh.
CAES plants are capable of producing power in the GW range with long power durations. However, the CAES plants are very expensive, on the order of ten of millions of dollars and take several years to build such a plant. The CAES plants are site-specific and may need natural gas as a fuel.
Recently, a new redox energy storage device was introduced by Electrochemical Design Associates, the Plurion Redox Battery. This battery uses mixed salts of zinc and cerium in methanesulfonic acid (MSA). The redox potential of this cell is an excess of 2 volts. This energy storage device based on zinc and cerium salts was discussed by B. J. Dougherty and co-workers at the Electrical Energy Storage Application and Technology meeting, 2002 EESAT meeting (http://www.sandia.gov/EESAT/). There is no mention of the composition of the cerium ion, (e.g., Ce+3 or Ce+4) in this paper.
MSA has been used in a variety of electrochemical process, most notably in electrodeposition (e.g., plating) applications. While offering advantages over other organic and mineral acids, MSA (and other sulfonic acids) and the purity and composition of the metal-sulfonate electrolyte must be uniquely balanced to ensure a quality metal coating and a high electrolytic efficient process.
The use of sulfonic acids in electrochemical applications, and in particular, MSA, is not new. Proell, W. A. in U.S. Pat. No. 2,525,942 claims the use of alkanesulfonic acid electrolytes in numerous types of electroplating. For the most part, Proell's formulations employed mixed alkanesulfonic acids of unspecified purity. In U.S. Pat. No. 2,525,942 Proell made specific claims for lead, nickel, cadmium, silver and zinc. In another U.S. Pat. No. 2,525,943, Proell specifically claims the use of alkanesulfonic acid based electrolytes in copper electroplating and the exact compositions and purity of the plating formulations were not disclosed. In a separate publication (Proell, W. A.; Faust, C. L.; Agruss, B.; Combs, E. L.; The Monthly Review of the American Electroplaters Society 1947, 34, 541-9) Proell describes preferred formulations for copper plating from mixed alkanesulfonic acid based electrolytes.
Martyak and co-workers in EP 0786539 A2 have discussed zinc deposition from MSA-based electrolytes. The acid electrolytes contained from about 5 grams per liter to about 175 grams per liter of the zinc-sulfonate salt. The pH described in this application claims the zinc sulfonate solutions operate best at a pH from about 2.0 or greater and preferably in the range from 3-5. The efficiency for zinc deposition is near 100% even at high current densities. Additives to the solution affected the quality of the zinc deposit. To minimize roughness in the zinc surface, it was necessary to use organic additives such as blocked and random co-polymers of alkylene oxides.
The use of cerium in sulfonic acids was the scope of inventions by Kreh and co-workers in U.S. Pat. Nos. 4,701,245 A1, 4,670,108 A1, 4,647,349 A1 and 4,639,298 A1. The oxidation of organic compounds in these patents was effected by the use of cerric compounds such as cerric methanesulfonate and cerric trifluoromethanesulfonate. In all cases, oxidation was complete by only using cerium ion in its highest oxidation state, Ce+4. The cerrous (Ce+3)/cerric (Ce+4) concentrations in MSA are critical in maintaining a stable oxidizing environment. The concentration of cerium discussed in U.S. Pat. No. 4,639,298 is at least 0.2 M but it does not differentiate between Ce+3 and Ce+4. Only the Ce+4 ion is an oxidant and necessary in the MSA solution to effect the oxidation reaction. The concentration of free MSA is also important to assist in dissolving the cerium compounds and the preferred concentrations of free MSA are from 1.5 M to about 9.0 M.
Thus, a new energy storage device based on zinc and cerium salts in a sulfonic acid electrolyte leads to many challenges. A cathodic reaction of zinc ion to zinc metal must be balanced by the oxidation of cerrous ion to cerric ion:Zn+2+2e−→Zno 2Ce+3→2Ce+4+2e−
For every mole of zinc ion reduced at the cathode, two moles of cerric ion are produced at the anode. Additional free acid is necessary in this process to impart conductivity to the redox system and thus lower the required voltage. However, as discussed in EP 0786539 A2, the pH for zinc deposition should be greater than 2.0 and preferably from 3-5. The concentration of zinc ion should also be high to achieve commercially acceptable deposition rates and smooth zinc coatings. Kreh and co-workers in U.S. Pat. No. 4,639,298 A1 discuss the use of high free sulfonic acid concentrations, greater than 1.5 M and preferably greater than 3.0 M. This high free sulfonic acid concentration necessary for cerium solubility, would result in a low pH, <1.0, and thus affect the zinc deposition process. The high free sulfonic acid concentration also affects the solubility ratio of Ce+3/Ce+4.
It thus would be desirable to have new electrochemical energy storage devices that contain compositions based on zinc and lanthanide series salts in sulfonic acids that produce high deposition efficiencies for zinc ion to zinc metal, free sulfonic acid to impart sufficient conductivity for the redox battery yet maintain high solubility of lanthanide series ion in solution to complete the redox couple.
It would be particularly desirable to have new sulfonic acid compositions that can be effectively used with metals of strong reducing capabilities such as zinc without deleterious effects such as odor often produced when using sulfonic acids containing impurities capable of producing odor.